42 The Dalhy Profile Recorder 



The position of the point Z must also be carefully marked on the 

 surface so that the exploring roller can be re-set to this point when 

 taking subsequent records of the surface. The re-setting to Z ensures 

 that the families of profile curves in all the series of records which may 

 be taken of the surface are recorded in relation to the same reference 

 plane. In other words by re-setting the roller to Z the position of the 

 reference plane in relation to the plane of the machine is maintained 

 substantially without change however often the sample is replaced in 

 the machine. In the time elapsing between two successive measure- 

 ments of a family of profile curves of the surface, warping slightly 

 changes the distances of the point Z on the surface from the clamping 

 points ; but the change is quite negligible in its influence on the position 

 of the reference plane if the point Z is initially chosen close to a clamping 

 point. In Fig. 1 for example, Z is taken as close to the clamp A, Fig. 4, 

 as possible. It would probably be better in a definite series of experi- 

 ments to take Z as near to the central clamping point as possible (C 

 in Fig. 4). Then the warping of the surface would have the least effect 

 in changing the relative positions of the reference plane through Z and 

 the plane of the machine. 



Warping and shrinking also change the relative positions of the three 

 clamping points themselves during lapse of time. The change is likely 

 to be slight and the effect on the relative position of the reference plane 

 and the plane of the machine negligible. To secure uniformity in 

 practice it is advisable to re-clamp a particular sample so that the central 

 clamp (like C Fig. 4) grips the timber at the same point in all the re- 

 settings of the series. The clamps A and B will then grip the sample 

 at points displaced from the original points by amounts due to shrinkage 

 in the linear dimension of the imaginary triangle formed by joining the 

 three points at which the sample was originally clamped. 



In cases where great accuracy is required the hole-slot-plane method 

 of clamping may be used. Assuming the timber to be hard enough to 

 bear clamping without appreciably indenting the surface, a conical hole 

 is formed in the timber to receive the conical point C (Fig. 2) ; a 

 V-groove pointing towards C is formed to receive the conical point B ; 

 and the surface of the timber rests on the conical point A. The timber 

 sample when clamped down on to these conical points is then fixed 

 relatively to the frame of the machine in the most accurate manner 

 possible. 



In cases where the timber is soft and therefore the conical sup- 

 porting points are likely to form indentations in the surface, metal 



